The present invention relates to a continuous isothermal process for preparing mononitrotoluenes with concentration of the resultant waste sulfuric acid and recycling of the concentrated waste sulfuric acid into the process.
Mononitrotoluenes are important intermediates for preparing optical brighteners, plant protection agents, and pharmaceutical products. Mononitrotoluenes are prepared, for example, on an industrial scale, by isothermal nitration of toluene. In this process toluene is reacted with a mixture of sulfuric acid and nitric acid (mixed acid, nitrating acid) (see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology Vol. 17, 4th edition 1996, xe2x80x9cNitrationxe2x80x9d and xe2x80x9cNitrobenzenes and Nitrotoluenexe2x80x9d).
The reaction produces a considerable amount of waste sulfuric acid that is polluted with organic compounds (for example, dinitrotoluenes or nitrated cresols) and as a result the acid must be worked up in a manner that is intensive with respect to process and costs. To avoid the production of waste acid, processes have been developed that comprise concentration of sulfuric acid, the concentrated sulfuric acid being freed from water and organic compounds and then recycled in a circulation process back to the nitration reaction.
DE 195 39 205 A discloses process parameters for the mononitration of aromatics, the mixed acids being adapted to the properties of the aromatic to be nitrated in such a way as to produce an approximately 70% strength waste sulfuric acid. In addition, the use of partially concentrated waste acids having a sulfuric acid concentration between 85% and 92% is described.
U.S. Pat. No. 4,772,757 describes a process for preparing nitrobenzene in which the resultant waste acid is concentrated to 75 to 92% and recycled back to the nitration process. Since toluene, due to the methyl group, compared with benzene, is markedly more oxidation-sensitive and has a tendency in the nitration to form by-products, applying the reaction conditions for the nitration of benzene to the nitration of toluene involves the expectation of an increase in the amount of unwanted by-products.
Due to the solubility of organic compounds in sulfuric acid, organic by-products such as oxalic acid or benzoic acid accumulate in concentrated waste acids that are recycled to the nitration reaction. In addition, accumulation of nitrosylsulfuric acid can occur. The decomposition of these organic by-products and the heat of decomposition released in the course of this reaction lead to unwanted decomposition of the reaction product nitrotoluene. In addition, there is an adverse effect on the space-time yield, since the organic by-products can react with the nitric acid used in oxidative degradation reactions and thus some of the nitric acid is no longer available for the actual nitration.
To avoid accumulation of organic by-products in the concentrated waste sulfuric acid, this can be highly concentrated (which is to say, can be concentrated to about 96%). Under these concentration conditions the organic by-products are destroyed. Nitrosylsulfuric acid can be removed from the concentrated waste sulfuric acid by blasting with sulfur dioxide.
A disadvantage of concentrating waste sulfuric acid to 96% is that concentration must be carried out at temperatures of approximately 250xc2x0 C., which, in industrial scale reactions, leads to an increased energy consumption. In addition, waste sulfuric acid that is concentrated to 96% must be diluted in an additional reaction step before recycling to the nitration reaction, the dilution generally being carried out by mixing with non-concentrated waste acid. Blowing out nitrosylsulfuric acid must be carried out in an additional process step, disadvantageously.
There was therefore a requirement for a continuous isothermal process for preparing mononitrotoluene that permits inexpensive concentration of the waste sulfuric acid with subsequent recycling into the nitration reaction in the context of a circulation process, without organic by-products accumulating in the concentrated sulfuric acid.
Surprisingly, a continuous process has been found for preparing mononitrotoluenes by reacting toluene with nitric acid and sulfuric acid under isothermal reaction conditions comprising
(a) feeding 75 to 93% strength sulfuric acid, 60 to 70% strength nitric acid, and toluene into a reactor,
(b) separating the crude nitrotoluene from the waste sulfuric acid at the reactor outlet,
(c) concentrating the waste sulfuric acid in a single-stage concentration to 75 to 93%, and
(d) recycling the concentrated waste sulfuric acid in a circulation back to the nitration reaction.